1. Field of the Invention.
The present invention relates to a corona discharge treating method and apparatus for improving the quality of surfaces of resin moldings, and more particularly, relates to a corona discharge treating method and apparatus for three-dimensional resin moldings having corners.
2. Description of the Related Art
Generally, polyolefin-series resin such as polypropylene or the like has such a property that painting materials, joining materials, printing materials or the like are hardly adhered onto the surface thereof, because the resin has a little number of polar groups. Accordingly, in the case where painting, joining, printing or the like, is applied to the surface of the resin, the quality of the surface should be improved by pretreatment in order to improve the adhesive property thereof.
Various methods, such as flame treatment, acid solution treatment, plasma treatment, corona discharge treatment and the like, are used as pretreatment for improving the adhesive property of the polyolefin-series resin with respect to painting materials, joining materials, printing materials or the like.
The plasma treatment is a useful method for activating the almost whole surface of a resin molding at once, and accordingly, the plasma treatment has produced excellent results as pretreatment before painting or joining treatment for automobile bumpers, automobile moldings, etc.
However, plasma treatment requires a reduced-pressure plasma gas atmosphere. Accordingly, plasma treatment has a disadvantage that large-scale and high-cost equipments, such as a vacuum tank, a vacuum pump, a valve mechanism, a carrier gas and the like, are required as well as a large time is required to reduce pressure within the vacuum tank. Furthermore, in plasma treatment, manufacturing processes should be made by batch processing, so that automatization thereof is difficult. Accordingly, plasma treatment has another disadvantage that the vacuum condition is interrupted for each manufacturing process to thereby require a long time for next reduction of pressure.
From this point of view, corona discharge treatment which does not require such equipments as a vacuum tank and the like, is very advantageous. Corona discharge treatment, however, suffers from the limitation that a discharge electrode should be always close to a surface to be treated. Accordingly, corona discharge treatment has been used only for film-like or plate-like resin moldings (hereafter generically referred to as two-dimensional resin moldings). There is no report or reference that suggests use of corona discharge treatment for resin moldings having three-dimensionally uneven surfaces to be treated (hereafter generically referred to as three-dimensional resin moldings).
It is to be understood that realization of corona discharge treatment for three-dimensional resin moldings produces a large advantage in the cost of equipment and the time required for the treatment. For the realization, however, several problems should be solved. One of the problems is the lack of uniformity in the state of improved quality. Because the surface of a three-dimensional resin molding to be treated is rough, the distance between the corona discharge electrode and the surface is changed. The power of discharge becomes higher as this distance decreases, while the power of discharge becomes lower as this distance increases. Accordingly, there is a possibility of producing the lack of uniformity in improved quality depending on the positions of the treated surface.